Hi All, I guess this is an alternative to Ard's series since he beat me to it. I'll benchmark his series so we have comparable numbers...
As I reported at [1], kstack offset randomisation suffers from a couple of bugs and, on arm64 at least, the performance is poor. This series attempts to fix both; patch 1 provides back-portable fixes for the functional bugs. Patch 2 proposes a performance improvement approach. I've looked at a few different options but ultimately decided that Jeremy's original prng approach is the fastest, and I want to make the argument that it is "secure enough" for this use case. This could be combined with an occasional reseed from the crng; If amortized over ~10K syscalls it makes no difference to mean performance. But I'm not convinced of the value. I think we should aim to keep it simple and architecture agnostic. Ultimately we are only randomising the offset with 6 bits. So an attack that depnds on knowing the stack offset would get lucky 1 in 64 attempts. Surely that's trivially brute forcable? Given that, why go to so much effort to ensure the 6 bits are strong random? More details in the commit logs. Performance =========== Mean and tail performance of 3 "small" syscalls was measured. syscall was made 10 million times and each individually measured and binned. These results have low noise so I'm confident that they are trustworthy. The baseline if v6.18-rc5 with stack randomization turned *off*. So I'm showing performance cost of turning it on without any changes to the implementation, then the reduced performance cost of turning it on with my changes applied. arm64 (AWS Graviton3): +-----------------+--------------+-------------+---------------+ | Benchmark | Result Class | v6.18-rc5 | per-task-prng | | | | rndstack-on | | | | | | | +=================+==============+=============+===============+ | syscall/getpid | mean (ns) | (R) 15.62% | (R) 3.43% | | | p99 (ns) | (R) 155.01% | (R) 3.20% | | | p99.9 (ns) | (R) 156.71% | (R) 2.93% | +-----------------+--------------+-------------+---------------+ | syscall/getppid | mean (ns) | (R) 14.09% | (R) 2.12% | | | p99 (ns) | (R) 152.81% | 1.55% | | | p99.9 (ns) | (R) 153.67% | 1.77% | +-----------------+--------------+-------------+---------------+ | syscall/invalid | mean (ns) | (R) 13.89% | (R) 3.32% | | | p99 (ns) | (R) 165.82% | (R) 3.51% | | | p99.9 (ns) | (R) 168.83% | (R) 3.77% | +-----------------+--------------+-------------+---------------+ Because arm64 was previously using get_random_u16(), it was expensive when it didn't have any buffered bits and had to call into the crng. That's what caused the enormous tail latency. x86 (AWS Sapphire Rapids): +-----------------+--------------+-------------+---------------+ | Benchmark | Result Class | v6.18-rc5 | per-task-prng | | | | rndstack-on | | | | | | | +=================+==============+=============+===============+ | syscall/getpid | mean (ns) | (R) 13.32% | (R) 4.60% | | | p99 (ns) | (R) 13.38% | (R) 18.08% | | | p99.9 (ns) | 16.26% | (R) 19.38% | +-----------------+--------------+-------------+---------------+ | syscall/getppid | mean (ns) | (R) 11.96% | (R) 5.26% | | | p99 (ns) | (R) 11.83% | (R) 8.35% | | | p99.9 (ns) | (R) 11.42% | (R) 22.37% | +-----------------+--------------+-------------+---------------+ | syscall/invalid | mean (ns) | (R) 10.58% | (R) 2.91% | | | p99 (ns) | (R) 10.51% | (R) 4.36% | | | p99.9 (ns) | (R) 10.35% | (R) 21.97% | +-----------------+--------------+-------------+---------------+ I was surprised to see that the baseline cost on x86 is 10-12% since it is just using rdtsc. But as I say, I believe the results are accurate. Thanks, Ryan Ryan Roberts (2): randomize_kstack: Maintain kstack_offset per task randomize_kstack: Unify random source across arches arch/Kconfig | 5 ++- arch/arm64/kernel/syscall.c | 11 ------ arch/loongarch/kernel/syscall.c | 11 ------ arch/powerpc/kernel/syscall.c | 12 ------- arch/riscv/kernel/traps.c | 12 ------- arch/s390/include/asm/entry-common.h | 8 ----- arch/x86/include/asm/entry-common.h | 12 ------- include/linux/prandom.h | 7 +++- include/linux/randomize_kstack.h | 53 ++++++++++------------------ include/linux/sched.h | 5 +++ init/main.c | 1 - kernel/fork.c | 2 ++ 12 files changed, 34 insertions(+), 105 deletions(-) -- 2.43.0
